Balanced detector for altimeters



Oct. A29, 1946. D. BLITZ BALANCED DETECTOR FOR ALTIMETERS Filed May so, 1944 Hrm/aver Patented Oct. 29, 1946 FFECE BALANCED BETECTR FR ALTEIVETERS Daniel Blitz, Princeton, N.

Corporation of America,

vare

l1., assignor to Radio a corporation of Dela- Appiication May 30, 1944, Serial No. 538,052

(Cl. Z50-27) Claims. l

My invention relates to balanced detectors for altimeters or other distance measuring systems of the frequency modulated or FM type.

In application Serial No. 445,720, led June 4, i942, in the name of Royden C. Sanders, Jr., entitled Frequency modulated altimeter or distance indicator, there is described a balanced beat frequency detector comprising diodes and a tuned line in which undesired amplitude modulation in an FM altimeter or radar system is balanced out. In any balanced detector that is employed in a system operating on high radio frequencies such as those utilized in radar systems, it is diiicult to obtain a perfect balance.

An object of the present invention is to provide an improved detector of the balanced type, and particularly to provide an improved method of and means for coupling to a balanced detector.

A further object of the invention is to provide an improved balanced detector for use in FM distance measuring systems such as altimeters,

A further object of the invent-ion is to provide a detector of the balanced type that can be adjusted to an exact bala-nce with a minimum of difnculty.

A still further object of the invention is to provide an improved coupling means for balanced detectors operating on centimeter Wave signals.

In one embodiment, the fention is applied to a balanced diode detector in which each detector tube has the reflected signal and the heterodyning signal applied thereto with the two signals having a phase relation with respect to each other at one tube that is the opposite of their phase relation with respect to each other at the other tube. The reflected signal may be applied to the detector tubes in parallel relation while the heterodyning signal from the transmitter is applied to push-pull relation through my improved coupling circuit which comprises a double coupling loop. The output signal may be `taken ofi the anode of one tube and olT the cathode of the other tube and the two output signals combined, whereby any amplitude modulation of the applied signals, and particularly of the heterodyning signal, may be balanced out. The heterodyne or diierence frequency signal, on the other hand, appears in the detector output cirm cuit. Likewise, any interfering or jamming signal may be balanced out.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure l. is a circuit and block diagram of an FM altimeter embodying the invention; Figure 2 picked up by a receiver is a View showing the mechanical arrangement of my double coupling loop employed in the receiver illustrated in Fig. 1; and Figure 3 is a View showing the relative positioning of the coupling loops to the tuned line of Fig. 1. In the several figures, like parts are indicated by the same reference characters.

Referring to Fig. 1, the invention is shown applied to the balanced detector of a radio altimeter. The altimeter comprises a frequency-modulated radio transmitter ill which radiates a frequency-modulated carrier wave from an antenna i i. Suitable means for sweeping the carrier Wave frequency through a certain frequency range periodically is indicated at I2.

The associated receiver includes a balanced detector which comprises a pair of diodes I3 and I4 that have signal applied thereto from a tuned line E6 having a grounded center conductor Il associated therewith. The reflected signal is antenna I8 and is supplied through a coaxial line IS to a coupling loop El which (as shown in Fig. 3) is positioned under the conductor Il with its plane at right angles to the plane of the tuned line I 5. Thus the reiiected signal is applied with like polarity to the anodes i5 and 2t of the diodes I3 and Ill. Stated differently, it is applied in parallel relation to the diodes.

rlhe same frequency-modulated signal that is supplied to the transmitter antenna II is also supplied as a heterodyning signal through either a coaxial or a two-wire line 253 to a coupling loop 2l which is made a double loop for the purpose of improving the circuit balance as explained hereinafter. The double loop 2l is positioned with its plane parallel to the plane of the tuned line I5 and it is centered with respect to the line It as illustrated. Thus the heterodyning signal is applied With opposite polarities to the anodes I5 aud gli of the diodes I3 and Ill, i. e., it is applied in push-pull relation instead of in parallel relation. The two applied signals produce a differor beat frequency signal which is a measure of the time required for the signal radiated from antenna Ii to reach antenna I8 by reection and which is, therefore, a measure of the distanceto the reilecting surface.

The tuned line l5 includes Itwo coaxial conductors 3| and 32, the inner conductors of Which are connected Ito the anodes Ill and 2d. The outer conductors ci the coaxial conductors 3i and 32 are connected at each end lto the inner conductor by means of capacitors 33, 34, 36 and 31 whereby 3 the inner and outer conductors are at the same potential at radio frequencies.

The cathode 33 of diode I3 is conductively connected to ground preferably through a low impedance resistor 33 and a capacitor llI in parallel. The cathode 12 of diode I6 is connected to ground at radio frequencies through a capacitor 13. The cathode 38 may be connected directly to ground, if desired, but the connection through capacitor 4I is preferred since it makes the circuit more symmetrical and more easily balanced. This is true because capacitor il at diode I3 corresponds to capacitor 43 at diode I4, the latter capacitor being required where the beat frequency signal is taken off the cathode. The resistor 39 is required to complete the direct-current path for diode i3. It may have a resistance of 1GO ohms, for example. Each side of the heater lament for cathode 38 is held at ground potential at radio frequencies by means of bypass capacitors lll and d6. Also, one side of the heater filament for cathode l2 is held at ground potential at radio frequencies by means of a bypass Q1, the other side of the filament being connected directly to ground.

Referring again to the tuned line i6, the ends of the outer conductors of lines 3! and 32 are connected to ground through a connection i8 and the center conductor I'I. An adjustable shorting bar figa, 49h is provided for tuning the two circuits iormed by each coaxial line and the center conductor il to the mid-frequency of the signal from the receiver antenna I8. It will be seen that there is one resonant circuit that may be traced from the :anode I5 through the capacitor 33, the conductor BI, the snorting bar section @3a and the conductor I'I to ground; and a second resonant circuit that may be traced from the anode 25J through the capacitor 3S, the conductor 32, the shorting bar section 49h and the conductor II to ground.

A third resonant path is provided by a capacitor 5I that is in adjustable contact with the outer conductors of coaxial conductors 3l and 32. This path may be traced from the anode I5 through the capacitor 33, the conductor 3i, the capacitor 5I, the conductor 32 through capacitor 36 to the anode 29. This third path is made resonant at the mid-frequency of the heterodyning or mixing signal from the transmitter, this mid-frequency being the same, of course, as the mid-frequency of the received reilected signal.

It will be seen that the lines of force from the loop 2l will cut the conductors 3l and 32 as indicated in Fig. 3 whereby the ow of transmitter or mixing signal in tuned circuit I6 will be as indicated by the arrows ir in Fig. l. Thus, the mixing signal at the anode l5 is 180 degrees out of phase with the mixing signal at the anode 2li. The lines of force from the loop ZI out the conductors 3l and 32 as shown in Fig. 3 whereby the flow of reflected signal will be as shown by the arrows iR in Fig. l. It will be seen that the reflected signal on the anode i5 is in phase with the reflected signal on the anode Eil.

The loops 2l and 2l are adjustable parallel to the plane of the lines BI and 32 to permit balancing adjustments to compensate for any difference in diode characteristics or in the physical construction of the tuned circuit Sii-32 or other parts of the receiver. The loop 2i preferably is terminated by a tuning capacitor 52. The double loop il may be supported by a block lili of insulating material as shown in Fig. 2 and covered by a rectangular shielding can l5 that is part of the receiver shielding (not shown). As shown in Figs. 1 and 2, one of the loops 2l is connected through a terminating resistor E@ to one side of the input line 26; the other end of this loop may be connected to ground through the shielding can 45. Likewise, the other of the loops 2l is connected through a terminating resistor 515 to the other side of the line El; and the other end of this loop is connected to the shielding can 45. lf desired, the terminating resistors 50 and 55 may be located `adjacent to the ground connections instead of in the locations illustrated.

The transmitter end of the line 2li preferably is grounded at its electrical midpoint 6@ for balancing out any electrostatic pickup from the transmitter because any signal thus picked up and supplied -to the detector will be in the wrong phase and will unbalance the detector. However, grounding the transmitter end of the line Z6 does not always eliminate signal due to capacity pick-up for the reason that the grounded point Sii is not always exact as to location. Any such pick-up current may have diiferent values in the two sides of the line 2li. It is because of this that the double loop 27 is employed. 1t will be apparent that the coupling to the tuned line 3I, 32 from 4any part of the doubleloop 2l will include both leads of the line 2li and any current difference will be averaged out.

Referring now to the detector output circuit, it will be seen that the beat frequency signal is taken ofi the anode l5 through the inner conductor of the coaxial line "al and supplied through a conductor a resistor 5G and a portion of a resistor 5l to a variable tap 58 and an output resistor 59. The beat frequency signal also is taken 01T the cathode f1.2 through a low impedance filter resisto-r @I (having a resistance of 100 ohms, for example) and supplied through a resistor G2 and the other portion of resistor 5l to the variable tap 53 and the output resistor 52. A filter capacitor d3 is connected between ground and the end of filter resistor 5I remote from the cathode. The resistor 5I and the capacitor 23 are provided to keep the radio frequency out of the output circuit. The beat or audio frequency circuit for diode id is completed from the anode 2G, through the inner conductor of coaxial Vline 32 and through a connection Eli to the grounded conductor Il.

From the foregoing, it will be evident that any amplitude modulation may be balanced out by adjusting the tap 5? since this modulation at one anode is in phase with that at the other anode. rPhe beat frequency signal, on the other hand, is 180 degrees out of phase at one anode with respect to the beat frequency signal at the other anode whereby these signals add due to phase reversal obtained the output connec-Y tions,

It may be noted, merely by way of example, that all the capacitors shown in Fig. l, except the tuning capacitor 52, have a capacity of 55 micro-microfarads. Also, resistors 56 and S2 may 5 be 50cc ohms each, resistor si be 250cc cams and resistor may be 47,060 ohms. These values are for a receiver operating at a 440 megacycle carrier frequency that is being swept through a range of 40 rnegacycles,

It should be understood that the mixing signal and the reflected signal may be applied to the detector in parallel relation and pushpuil relation, respectively, if desired instead of in the opposite way described above.

I claim as my invention:

1. In combination, a baia-need circuit comprising a tuned line, a two-wire line for supplying signal from a signal source to said tuned line, said two-wire line being grounded adjacent to said signal source at substantially the electrical midpoint of said two-wire line, and means for coupling said two-wire line to said tuned line, said means comprising a double inductive loop coupled to said tuned line, said double loop having two sides that are adjacent to the two conductors, respectively, of said tuned line, the midpoint of said double loop being grounded whereby each side of said double loop includes conductors that carry the current ow of each of the conductors of said two-wire line so that any coupling due to unequal current iiow in opposite sides of said two-wire line is balanced out.

2. In combination, a balanced detector comprising a tuned line and a pair of rectifiers coupled to opposite sides, respectively, of said line, a two-wire line for supplying signal from a signal source to said tuned line, said two-wire line being grounded adjacent to said signal source at substantially the balance point of said two-wire line, and means for coupling said twowire line to said tuned line, said means cornprising a double inductive loop coupled to said tuned line, said double loop having two sides that are adjacent to the two conductors, respectively, of said tuned line, the midpoint of said double loop being grounded whereby each side of said double loop includes conductors that carry the current flow of each of the conductors of said two-wire line so that any coupling due to misplacement of said grounded point of the two-wire line is balanced out.

3. In combination, a balanced circuit comprising a tuned line, a, signal source, a two-wire line for supplying signal from said signal source to said tuned line, said two-wire line being inductively coupled to said signal source through a coupling loop which is grounded at substantially its balance point, and means for coupling said two-wire line to said tuned line, said means comprising a double inductive loop coupled to said tuned line, said double loop having two sides that are adjacent to the two conductors, respectively, of said tuned line, the midpoint of said double loop being grounded whereby each side of said double loop includes conductors that carry the current flow or" each of the conductors of said two-wire line so that any coupling due to misplacement of said grounded point of the two-wire line is balanced out.

4. A balanced detector comprising a tuned line having an electrical midpoint, a pair of rectiiers coupled, respectively, between said midpoint and opposite sides of said line, a multiple turn coupling loop coupled to said tuned line, a pair of conductors for supplying signal to said multiple turn coupling loop, said conductors being inductively coupled through a coupling loop portion to a signal source, said conductors being connected to ground at a balance point on said coupling loop portion, said multiple turn coupling loop having two sides that are adjacent to the two conductors, respectively, of said tuned line, each side of said multiple turn loop including a pair of wires carrying the current of each of said conductors, respectively, of said pair.

5. In a frequency-modulated radar system, a balanced detector comprising a tuned line having an electrical midpoint, a pair of rectiers coupled, respectively, between said midpoint and opposite sides of said line, a double loop coupled to said tuned line, a pair of conductors for supplying signal to said double coupling loop, said conductors being inductively coupled through a coupling loop portion to a, signal source, said conductors being connected to ground at a balance point on said coupling loop portion, said double coupling loop having two sides that are adjacent to the two conductors, respectively, of said tuned line, each side of said double loop including one oi said conductors of said pair whereby differences in current flow in the conductors of said pair are averaged out.

DANIEL BLITZ. 

